Our invention relates to an apparatus and a method for molding precisely shaped articles such as ophthalmic lenses from synthetic resin.
Articles such as ophthalmic lenses must be formed as a bubble free body having accurately contoured optically smooth outer surfaces. In the interest of safety and cost reduction, it is desirable that such lenses be formed from a synthetic resin to provide a lens having the required ophthalmic properties and which is substantially unbreakable, which weighs less and which will withstand wear and abuse at least to the degree which a glass lens will stand wear and abuse. While other synthetic resins may be suitable for the formation of such lenses, we have found that polycarbonate resins fulfill the requirements for a synthetic resin lens outlined above.
While it is desirable that ophthalmic lenses be molded from a synthetic resin such as a polycarbonate, the achievement of this result involves a number of problems. Upon being heated to a point at which it melts polycarbonate expands at a constant rate until it reaches a "glass transition" temperature. During this time, the molecules are absorbing energy yet neither the distance between the molecules, nor the energy level attained by the molecular particles is sufficient to allow for intermovement or change in physical state of the material. When the polycarbonate reaches its "glass transition" temperature, it attains a physical state in which the energy absorbed by the molecules is sufficient to allow movement within the molecular lattice. At this point, softening occurs and the material enters a viscous state. As the material is further heated, its density decreases at a sharply increased rate. The voids in the molecular lattice become greater and the molecules, activated by the elevated temperature to a higher energy level, move about more freely. Hence, the material with increase in temperature from the "glass transition" point will exhibit less and less viscosity until at about 500.degree. F. it is a relatively fluid somewhat viscous mass. Owing to the fact that polycarbonate is amorphous, it does not exhibit the characteristic of a sharp melting point and fluidity at the same temperature with heat of fusion required to melt and the removal of this heat of fusion to cause solidification at one temperature. Stated otherwise, there is some hysteresis in the process of heating up polycarbonate to a point at which it melts and then reducing its temperature so that it resolidifies.
A plot of relationship between temperature and specific volume of polycarbonate shows that above the "glass transition" point of about 305.degree. F., expansion with increased heat or contraction by the removal of heat occurs at a greatly accelerated rate than at temperatures below 305.degree. F. Shrinkage resulting from cooling in this zone occurs in the viscous state, is totally random in nature because molecules are free to move wherever they will in the molecular lattice and is of greater magnitude than in the zone below the glass transition point. Shrinkage in the zone below the glass transition point is linear and at a lower rate than in the zone above the glass transition point. In this zone, the molecules are locked into a rigid lattice structure and shrinkage behaves in the same manner as for any solid material.
In conventional injection molding practice, parts are so designed that part thickness differentials are kept to a minimum to avoid "sinks" resulting from the solidification of thinner portions before the solidification of thicker portions. In the design of lenses the only permissible criteria is perfection of curvature and yet "sinks" due to thickness differential must be avoided although thickness differential is appreciable in a majority of lenses.
Having the foregoing in mind, it will readily be appreciated that in the molding of ophthalmic lenses from a synthetic resin such as polycarbonate regardless of thickness differential, the material placed in the mold must attain the "glass transition" temperature evenly so as to minimize localized shrinkage and maintain "distortion" within acceptable limits.
Another problem which exists in the molding of ophthalmic lenses from synthetic resin occurs in the case of "high minus" lenses. In the prior art, as the material is injected into the mold at one location around the periphery of the finished lens, it tends to flow first toward the center and then down and around the mold sides toward a diametrically opposite location at which the material meets itself along a line. The result of this action is the formation of a "weld line" at the location at which the material being injected meets itself.
In another method resin under high pressure is injected into the space between dies mounted on powerful presses so that excess plastic is forced into an overflow chamber. This method requires huge pressures, since it does not provide for properly controlled cooling to prevent jamming by prematurely solidified portions of the injected plastic. The control of thickness of the resultant lens is made difficult by the overflow. The pressing operation has to be controlled to prevent overflow motion from causing strain within the plastic as it is on the verge of setting. The overflow plastic must be removed from the finished optical article. In addition to the foregoing, no one has yet produced high minus lenses without weld lines.